An optical device that uses a cholesteric liquid-crystal composition showing a cholesteric phase at room temperature by adding a chiral agent to a nematic liquid crystal is known. Such an optical device is generally formed in which the cholesteric liquid-crystal composition is interposed between a pair of substrates with electrodes having at least one electrode being a transparent electrode, and a display is performed by controlling liquid-crystals in planer (or Grandjean) alignment, focalconic alignment or homeotropic alignment when a voltage (driving voltage) is applied between the electrodes. In particular, at the planar alignment, a color display is allowed when light having a specific wavelength is reflected by selective reflection and a selective reflection wavelength is controlled in a visible light region. Light that enters in parallel to the helical axis of the planar alignment is separated into right-handed and left-handed circularly polarized light, and light in a direction identical with the helical direction of the cholesteric liquid-crystals is selectively reflected, and the peak wavelength of the reflected light is provided by an equation: λ0=n×p. Herein, p is the helical pitch (or pitch), and n is the average refractive index. Accordingly, if the pitch is changed by controlling the addition amount of the chiral agent, light having various wavelengths can be selectively reflected.
Memory properties can be provided for the cholesteric liquid-crystal composition. As a technique for providing the memory properties, the surface stabilized cholesteric texture (SSCT), the polymer stabilized cholesteric texture or the like is known. In an optical device using such cholesteric liquid-crystals having the memory properties, the voltage may be applied only upon rewriting the displays, and therefore the electric power consumption is low.
The optical device using the cholesteric liquid-crystals includes a liquid-crystal composition having suitable physical properties. In order to improve characteristics of the optical device, the liquid-crystal composition preferably has suitable physical properties. General physical properties necessary for a liquid-crystal compound being a component of the liquid-crystal composition are as described below:    (1) being chemically stable and physically stable;    (2) having a high clearing point (phase transition temperature between a liquid-crystal phase and an isotropic phase);    (3) having a low minimum temperature of the liquid-crystal phase;    (4) having an excellent compatibility with other liquid-crystal compounds;    (5) having a dielectric anisotropy of a suitable magnitude; and    (6) having a refractive index anisotropy of a suitable magnitude.
From the viewpoint of reducing the driving voltage of the cholesteric liquid-crystals, a compound having a large dielectric anisotropy is preferred. From the viewpoint of expanding the selective reflection wavelength band, a compound having a large refractive index anisotropy is preferred.
If a liquid-crystal composition containing a liquid-crystal compound being chemically and physically stable as described in property (1) is used for a liquid-crystal display device being one kind of optical device, the voltage holding ratio can be increased.
In a liquid-crystal composition containing a liquid-crystal compound having the high clearing point or the low minimum temperature of the liquid-crystal phase as described in properties (2) and (3), the temperature range of the cholesteric phase can be extended, and the liquid-crystal composition can be used in a display device in a wide temperature range. In order to develop characteristics that are difficult to be achieved by a single compound, the liquid-crystal compound is generally used in the form of a liquid-crystal composition prepared by mixing the compound with many other liquid-crystal compounds. Accordingly, the liquid-crystal compound to be used for the liquid-crystal display device preferably has the good compatibility with other liquid-crystal compounds as described in property (4). Furthermore, a liquid-crystal composition having a low driving voltage is required for the liquid-crystal material to be used. A liquid-crystal compound having the large dielectric anisotropy is preferably used in order to drive at a low voltage the optical device to be driven in the cholesteric phase, and a liquid-crystal compound having the large refractive index anisotropy is preferably used in order to expand the selective reflection wavelength band.
WO 2010/058681 (Patent literature 1) discloses an optically isotropic liquid-crystal composition containing a compound represented by formula (1-1), but any of the liquid-crystal compositions each containing a chiral agent described in Examples in Patent literature 1 has a selective reflection wavelength of the cholesteric phase in the range of less than 400 nm at ordinary temperature (25° C.)